This invention relates to a method for the cooling of outer shrouds of rotor blades of a gas turbine which are subject to a hot-gas flow downstream of stator blades of the gas turbine and which are passively cooled with cooling air supplied via a turbine casing, as well as to an apparatus for the implementation of the method.
The efficiency of gas turbines can, as is generally known, be enhanced by increasing the turbine inlet temperature, which, however, is limited by the materials available for the stator and rotor blades of the high-pressure turbine. The turbine inlet temperature can, however, be increased if the cooling of the respective components is improved. As is generally known, the temperature of the disks, blades and platforms or shrouds, as applicable, is limited by active cooling using cooling air fed through inner ducts and partly by film cooling air fed-via-film cooling air holes to the outer surfaces.
Furthermore, in order to improve the cooling effect, the upper platforms or shrouds of the stator blades and the rotor blades are cooled passively, i.e. by means of compressor air supplied to these components from the outside. Passive cooling of the upper shrouds of the rotor blades is, however, inadequate in that the cooling air, which is supplied axially to the shrouds via an annular gap between the platforms of the stator blades and the turbine casing (e.g. the liner), primarily flows to the upper surface of the shrouds axially above the separation flow line of the hot-gas flow. This means that the cooling-air flows via the labyrinth seal provided between liner and shrouds, while the thermally highly loaded bottom surface of the shrouds is cooled only locally, while the thermally most loaded areas, such as the fillet radii between blade and shroud, can be missed entirely. The application of active cooling methods (internal cooling) to increase the cooling effect entails, on the one hand, considerable investment for the manufacture of shrouded rotor blades, and on the other hand, an increase in weight from the higher wall thicknesses required in such case. Passive cooling of the rotor blade shrouds can also be improved by substantially increasing the cooling air mass flow, however, with the disadvantage of high aerodynamic losses.